Some aircraft use counter rotating propellers on opposite wings. For example, the engine mounted on the left wing may have a clockwise rotating propeller while the engine mounted on the right wing has a counterclockwise rotating propeller. The air entering the engine must first flow through the propeller whose pumping action swirls the air in the direction the propeller is rotating. A disadvantage to this counter rotating configuration is that for at least one engine the propeller rotates in the same direction as its internal components. Under these conditions the propeller imparts positive swirl to the entering air. Positive swirl is swirl in the direction of rotation of the engine's compressor, and results in the engine's compressor producing lower flow and pressure ratio ,than when the propeller imparts negative swirl. Negative swirl is swirl in the direction of rotation opposite that of the engine's compressor.
Accordingly, there is a need for an turboprop engine in which the airflow entering the compressor has negative swirl regardless of the direction of propeller rotation.